This invention relates to Josephson junction devices and more particularly to a single-crystal, silicon barrier, Josephson junction and method of making same.
Heretofore various processes have attempted to make Josephson junctions by use of semiconductors. A number of these processes have made use of deposited semiconductor films which have resulted in devices with very low current density and nonreproducible properties. Deposited films usually end up with pinholes in the deposited barrier and since the films are either amorphous or polycrystalline, doping cannot be adequately controlled.
Josephson junctions with good electrical performance have been processed by use of bulk silicon barriers. One such configuration is made from a silicon wafer which has been heavily doped on one surface. The silicon is etched away except in the boron doped area and then superconducting films are deposited onto each surface of the boror doped silicon which forms the barrier. This structure has the disadvantage that a large wafer area is used for each junction and the junction contacts are on opposite sides of the wafer which makes it almost impossible to fabricate superconducting integrated circuits or Josephson junction detector arrays.
In another configuration, a strip of superconducting film is deposited onto a silicon substrate. A break is then formed across the superconducting film so that the only conduction path across the break is through the silicon substrate; therefore, the tunneling barrier is single-crystal silicon. This configuration is compact and planar so that integrated circuits and detector arrays are feasible. However, the break in the superconducting film must have a very narrow width approximating 1000 A to 3000 A and these line widths require electron beam or other high-resolution lithography.
Solutions suitable for etching silicon have been set forth in an article "The Etching of Deep Vertical-walled Patterns in Silicon", by A. T. Stoller, RCA Review, pp. 271-275, June 1970. This article also sets forth that vertical walls may be formed by properly oriented single-crystal silicon. Another article, "Ethylene Diamine-Pyrocatechol-Water Mixture Shows Etching Anomaly in Boron-Doped Silicon", by A. Bohg, J. Electrochemical Society, Vol. 118, No. 2, pages 401-402, 1971, reports another solution for etching silicon. The latter article sets forth that silicon doped with a concentration of boron greater than 7.times.10.sup.19 cm.sup.-3 resisted etching by the solution.